Thinned, back illuminated, semiconductor imaging devices are advantageous over front-illuminated imagers for high fill factor and better overall efficiency of charge carrier generation and collection. A desire for such devices is that the charge carriers generated by light or other emanation incident on the backside should be driven to the front side quickly to avoid any horizontal drift, which may smear the image. It is also desirable to minimize the recombination of the generated carriers before they reach the front side, since such recombination reduces overall efficiency and sensitivity of the device.
These desires may be achieved by providing a thin semiconductor layer and a high electric field within this layer. The field should extend to the back surface, so that the generated carriers, such as electrons or holes, can be driven quickly to the front side. This requires additional treatment at the backside of the device, which adds to complexity of the fabrication process. One current technique includes chemical thinning of semiconductor wafers and deposition of a “flash gate” at the backside after thinning. This requires critical thickness control of the backside flash gate. Another technique involves growth of a thin dopant layer on a wafer back using molecular beam epitaxy (MBE). Still another known method used to provide a desired electric field is to create a gradient of doping inside the thinned semiconductor layer by backside implant of the layer followed by appropriate heat treatment for annealing and activation.
These methods can not be easily included in conventional semiconductor foundry processing, and require more expensive custom processing. They are therefore often not cost-effective and not suitable for commercial manufacturing.
Back-illuminated imaging devices may be designed to operate at wavelengths ranging from less than 100 nanometers (deep ultraviolet) to more than 3000 nanometers (far infrared). An important factor that affects the sensitivity of back illuminated imagers is the absorption depth of radiation in the semiconductor bulk. In general, the radiation will be absorbed within a region close to the back surface of the device. For maximum device efficiency, all charge carriers generated in this region must reach optical detection components situated on the opposing front side of the device. A general method that is employed to increase the sensitivity of a thinned back-illuminated imager is to implant p-type or n-type dopant at the backside and, with later heat treatments, create a dopant concentration profile which decreases in the direction toward the front side of the thin substrate. In the case of p-type doping, such doping concentration gradient gives rise to an electric field tending to drive light-generated electrons toward the front side. In the case of n-type doping, such doping concentration gradient gives rise to an electric field tending to drive light-generated holes toward the front side.
Another problem encountered with the design and operation of back-illuminated imagers is the presence of dark current. Dark current is the generation of carriers (electrons or holes), exhibited by a back-illuminated imager during periods when the imager is not actively being exposed to light. Dark current is detrimental to back-illuminated imager operation because excess dark current signal collected along with a desired optically generated signal results in higher levels of fixed pattern and excess random shot noise. The offset signal produced by dark current is seen as a non-uniform shading in a displayed image. Assuming that an imager employs signal electrons, the excess dark current noise is proportional to the square root of the number of dark current electrons in a pixel. Since dark current noise is uncorrelated with other imager noise sources, the noise due to dark current adds to the overall noise from other sources in quadrature. Dark current noise increases with increased temperature.
Accordingly, what would be desirable, but has not yet been provided, is a device and method for effectively reducing the amount of dark current reaching front imaging components in back-illuminated imaging devices.